The invention relates to an optical amplifier comprising a solid-state laser for amplifying an optical input signal to produce an optical output signal, and a power supply source connected to the solid-state laser for supplying electrical pump energy to the solid-state laser.
The invention also relates to a transmission system utilizing such amplifiers.
An optical amplifier as defined in the opening paragraph is known from the journal article "Compensation of non-linearity in semiconductor optical amplifiers" by A. Saleh, R. Jopson and T. Darcie in Electronics Letters, July 1988, Vol. 24, No. 15, pp. 950-952.
Optical amplifiers are used, for example, in optical transmission systems which utilize glass fibers as a transmission medium. Optical transmission systems are often used for digital trunk lines between telephone exchanges. Another application is the use in cable television networks for the distribution of analog TV signals over large distances.
The maximum length of a glass fiber which may be covered at one time by an optical signal is restricted by various causes. A first cause for this restriction is an attenuation of the optical signal in the glass fiber transmitted by a transmitter. A second cause is a dispersion of the glass fiber which causes distortion of pulses transmitted by a transmitter in the case of digital transmission. This pulse distortion occurs as a result of the glass fiber producing a delay, which is different for optical signals having different wavelengths. This pulse distortion is proportional to the length of the glass fiber and, in addition, depends on the spectral width of the transmitted optical signal.
To cover large distances by means of glass fibers, what are commonly referred to as regenerative repeaters are used, and they are installed equidistantly in the glass fiber. A regenerative repeater comprises a complete optical receiver which converts received optical signal(s) into digital electric signal(s). In addition, a regenerative repeater comprises a complete optical transmitter which reconverts the digital signal(s) into light signal(s) for further transmission through a glass fiber. Such a regenerative repeater is rather complex.
In transmission systems utilizing lasers that generate an optical signal with a very small spectral bandwidth, the maximum distance that can be covered will primarily be determined by the attenuation of the glass fiber. In these case there is no need to use a regenerative repeater, for an optical amplifier will be sufficient to amplify the optical signal.
Because an optical amplifier, often comprising a solid-state laser as its amplifying element, is much simpler than a regenerative repeater, a considerable saving of costs can realized in using it instead of a regenerative repeater. A further advantage of the use of an optical amplifier is that its operation is independent of the symbol rate, and the amplifier is capable of simultaneously amplifying optical signals having different wavelengths.
An important property of an optical amplifier is its linearity of the relation between the optical input signal of the amplifier and the optical output signal. In trunk lines in which various amplitude-modulated optical carriers are utilized, what is commonly referred to as saturation-induced crosstalk occurs. This crosstalk occurs because the actual optical output power per carrier depends on the actual output power of the other carriers due to the non-linearity.
Another use of optical amplifiers which demands a proper linearity is the use of optical distribution in cable TV systems. In these systems the signal to be distributed, which may comprise, for example, the entire UHF band (470-860 MHz), is converted into an amplitude-modulated optical signal which is distributed through a glass fiber. If non-linear elements such as, for example, a non-linear optical amplifier, are present in this distribution system, noise components resulting from intermodulation may be generated and occur in a desired (TV) channel.
In the optical amplifier known from above journal article, a solid-state laser is used as an amplifier element. The electrical pump energy necessary for the solid-state laser is supplied by a power supply source in this case a current source. To improve the linearity of the solid-state laser, the current supplied to the solid-state laser contains not only a DC component but also an additional component which is proportional to the power of the light signal at the input of the solid-state laser.
To generate the additional component of the current supplied to the solid-state laser, an optical coupler is necessary for extracting part of the optical signals at the input of the solid-state laser. This extracted part of the optical signal is subsequently to be converted, by means of a photodiode into an electric signal, which, in turn, is reconverted into the additional component of the current for the solid-state laser. The presence of a
In trunk lines in which various amplitude-modulated optical carders are generating unit for generating the additional component of the current for the solid-state laser, makes the optical amplifier rather complicated.